The brushless electromotors generally have a cylindrical rotor provided on the outer peripheral surface thereof with permanent magnets made of ferrite or the like.
FIG. 28 shows a conventional brushless electromotor. The brushless electromotor 1 includes a motor casing (i.e. stator) 2 which consists of a cylindrical side wall 3, and a front face plate 4 and a rear face plate 5, both of which are employed to close opposite ends of the side wall 3. Inside the side wall 3, there are provided a plurality of driving coils 6 arranged in a cylindrical form and fixed to the inner surface of the side wall 3. A rotary shaft 8 is fixed concentrically to a rotor 7. The rotary shaft 8 projects from opposite ends of the rotor 7 so that it is supported at one end in a bearing 10 held in an opening 9 of the rear face plate 5 and at the other end in a bearing 12 held in an opening 11 of the front face plate 4. An annular member 13 is provided inside the side wall 3 of the motor casing 2 to support a plurality of magnetic pole sensors 14 closely adjacent to one end surface of the rotor 7.
In FIG. 29, the rotary shaft 8 is inserted into and integrated with a cylindrical yoke 70. The cylindrical yoke 70 carries on its outer peripheral surface a pair of arcuate permanent magnets 71 magnetized to have N-poles on their outer sides and S-poles on their inner sides and another pair of arcuate permanent magnets 72 magnetized to have S-poles on their outer sides and N-poles on their inner sides. The respective pairs of permanent magnets 71 and 72 are alternately arranged around the yoke 70 and bonded thereto.
In this brushless electromotor 1, the magnetic pole sensors 14 detect positions of the magnetic poles of the rotor 7 and, in response thereto, a control circuit (not shown) supplies the corresponding driving coils 6 with electric current so that an interaction of electric current and magnetic flux causes the rotor 7 to be rotated. As it has been rotated in this manner, the rotor 7 now presents new magnetic pole positions to be detected by the magnetic pole sensors 14 again, and the control circuit supplies the other driving coils 6 with electric current, causing the rotor 7 to be rotated again. Such operation is repeated and thereby the rotor 7 is continuously rotated. The rotary force thus generated is taken out as a motive power from the electric motor by way of the rotatable shaft 8.
FIG. 30 shows another rotor 7, in which the permanent magnets 71 and 72 are covered with a protective member 73 of nonmagnetic metal to prevent these permanent magnets 71 and 72 from flying off due to centrifugal force as the electromotor 1 is rotated at high speeds.
However, the maximum energy product and the residual flux density in the brushless electromotor utilizing the ferrite magnets are 3.3 MGOe and 2.8 KG, respectively, which are so low that it is necessary to increase the permeance of the magnetic circuit to develop a sufficient torque to drive the electromotor. As a result, it is necessary to use a large amount of magnets, making the electromotor disadvantageously bulky.
When the electromotor is used in the scroll type compressor or the like for high speed rotations, the permanent magnets can be destroyed or flying off because the stress generated by the centrifugal force becomes greater than the material strength of the permanent magnets and the adhesion of the magnets to the rotor.
Furthermore, covering the rotor with the protective member to avoid the flying-off of the permanent magnets not only complicates the manufacturing process of the rotor but also increases the gap between the rotor and the stator by the thickness of the protective member, and correspondingly increases the magnetic resistance. In a consequence, the magnet density decreases and the efficiency is significantly lowered.
The present invention has been developed to solve such problems as described above with respect to the conventional brushless electromotor.
Accordingly, it is an object of the invention to provide a compact rotor for use in the brushless electromotor, which can be manufactured efficiently and rotate at high speeds without the danger that the permanent magnets are destroyed or fly off during high speed rotations.